(1) Field of the Invention
(2) Description of Related Art
This invention relates to an apparatus and method for measuring vertical movement by using inertial sensors, and in particular to an apparatus and method for inertial sensing of human centre of mass movement in the vertical direction.
The centre of mass (CoM) is an important aspect of human locomotion and it has long been estimated that measurements of the vertical displacement of the centre of mass of a human, represents over 65% of the total mechanical work done in gait, i.e., whilst walking.
Reasonably accurate estimations of a location of a centre of mass of a human body have existed since the 19th Century from dissection of cadavers. This location has been measured more accurately in the 20th Century, for example using force plates, and is generally found to be around the lower back.
Measurements of the displacement of a person's centre of mass are found to provide very useful information for studying a person's movement. This can be useful in clinical care, for example in the study of the elderly or disabled, and in particular in determining the effects of conditions which affect walking. Measurements of centre of mass movement in gait before and after surgery can be used to measure the surgery's success and to direct suitable physiotherapy.
Measurement of the movement of the centre of mass of a human is also considered to be very useful in study of athletes and can be used in coaching and for developing improved technique of the movement of athletes.
Measurement of the displacement of the centre of mass can show asymmetry in the manner in which someone walks. In addition even where the significance of the data in isolation is not well understood, simply comparing the patterns of centre mass displacement found in people of an ordinary gait with those of untypical walking patterns or in an effective Olympic athlete, provides useful feedback and allows alterations to be made.
Desired uses for a gait assessment tool are to assist in the diagnosis of a gait abnormality, to monitor patient progress, to take a baseline assessment and to assist with orthotic prescription.
The best present systems for biomechanical studies and for measuring kinematic data from humans are considered to be optical motion camera systems (OMCS). OMCS determine the position of an object in a three dimensional calibrated space. These systems include passive and active marker systems. For example the Vicon system produced in Oxford, UK has infra red reflecting markers attached to an object to contain kinematic data from cameras, whilst the CODA system used by Codamotion in the UK uses active LED sensors applied to the object to obtain similar data. These systems provide very effective and accurate data but suffer from a number of disadvantages. Firstly, and perhaps most significantly, they are very expensive. Secondly the measurements are restricted to movement within calibrated areas. This typically means that only a few strides can be measured at one time, thereby not permitting study of all types of human locomotion. Additionally it is advantageous to measure movement in outdoor experiments in an attempt to replicate movement in the real world. Outdoor measurements are difficult with OMCS systems due to the need for calibrated areas and are particularly difficult for many of the common systems that use infra red reflective light that struggle with too much ambient sunlight.
A cheaper alternative to using OMCS systems is to use force plates to measure the ground reaction forces and to attempt to calculate the centre of mass displacement form the ground reaction forces. Whilst this is of some use, it is found that ground reaction forces are not the same as the centre of mass displacement. In addition force plates are still expensive for the amount of measurement that can be obtained. Accurate force plates are also immobile and need to be placed on a solid flat surface. They are small and require a subject to ensure that they place their foot onto the forceplate and so may affect walking style. For accuracy they also require floors, walls and connections to be specifically made to have a vibrationless signal.
Accelerometers have also been used in movement analysis, particularly in pedometers which measure the number of strides a person takes. One difficulty with using accelerometers to measure human locomotion is that the circular paths created by human locomotion mean that the angle of the axis of the accelerometer is constantly changing. The twisting of the axis makes it very difficult to calculate accelerations in the global reference frame, i.e., in terms of up and down against the force of gravity. Postural alignment of the walking subject and inaccuracy in positioning of the instrument must be corrected for the static gravity in order to assess true dynamic acceleration in gait.
There also exists units, such as inertial measurement units, that combine accelerometers with gyroscopes and possibly also with magnetometers. These measure changes in orientation as well as accelerations in the local axes of the accelerometers. These therefore provide the vectors of acceleration of the local object system which potentially could be transposed into axis of another system. For these reasons a combination of accelerators, gyroscopes and in some cases industrial micro electro-mechanical (MEM) IMUs (typically used for vibration sensing) have been used to measure the change of angle in joints and in other aspects of human movement. There has also been an attempt to estimate the centre of mass displacement of horses using an IMU. This was reasonably successful given that millimeter accuracy is not required for typical study of horses and it has now been realised because the IMU located on the back of a horse stays reasonably horizontal the majority of the time reducing the likelihood of error in the Z axis resulting from mathematical gimbal lock.
It has been commonly assumed that using known accelerometers, gyroscopes and inertial measurement units on the back of a human to measure displacement in the global frame will not be possible for providing any useful degree of accuracy. This is based on the fact that whilst the IMUs do give accurate data for the accelerations and orientation movement regarding their local frame, there will be a very significant drop in accuracy in attempting to translate this information into movements in the global frame and a further loss of accuracy in moving from the measured acceleration to measures of speed and position which are the more common parameters required by clinicians and sports coaches. The axes of any unit near the Centre of Mass will turn and twist greatly making translation to a global reference frame difficult.
It is now realised by the inventors of the current invention that much of the inaccuracy in measuring vertical centre of mass displacement from an IMU comes from the fact that the IMU is placed on the lumbar spine so much of the data comes from the IMU being rotated through 90 degrees from its intended orientation and that use of an Euler angle rotation matrix leads to much of this data being lost due to mathematical gimbal lock. Further it has been surprisingly found that through correct use of suitable techniques it is possible to not only provide accurate measurement of vertical acceleration in the global reference frame but also speed and changes in position in the vertical direction. The accuracy can be almost comparable to that of optical motion camera systems.
Surveys of physiotherapists, find that in clinical practise only a small minority of respondents had actually had a patient assessed in a gait laboratory. Reasons for not using a gait assessment tool include lack of time, budget constraints and lack of space-all of which are due to the requirement of OMCS. Instead the physiotherapist mainly relies on Visual Gait Assessment Scales (VGAS) which are subjective and sometimes unreliable as they are reliant on an individual assessor's opinion of the abnormality and its severity.
It is also desirable to calculate how much energy a person (or animal) has used whilst walking or running. This information is useful for clinical purposes and also for anyone exercising who wished to calculate the number of calories used. Typically this is estimated by use of a pedometer and knowledge of the person's weight. The calculation are then simply based on weight and number of strides and take no account of the efficiency of gait, or of walking speed, or terrain. It has been realised that because such a large part of the energy used during walking is from moving the centre of mass vertically (against gravity), that measurements of the movement of centre of mass can be used alongside activity measures, such as step counts, to more accurately determine the amount of energy used running or walking. Conventional measurement equipment such as OMCS are of limited use for this. They can only measure in the calibrated space and therefore could only obtain useful information about energy consumed for a person over a very limited range or on a treadmill—where the speed of travel and inclination are already known and there is less pressing need for more accurate energy calculations. Further force and energy used can be based partly on accelerations or speed of the centre of mass and the OMCS provide data relating to position, which would have to be differentiated thereby reducing accuracy.